Reef Tank Water Parameters: The Numbers That Actually Matter

Reef chemistry looks intimidating from the outside. It is not. Corals ask for a short list of numbers held steady — and steady matters far more than perfect.

Walk into any reef forum and you will find people agonizing over whether their alkalinity should be 8.2 or 8.4 dKH. Meanwhile their tank swings a full point every week because they dose by feel and test when they remember. The second problem will kill corals. The first one never will. That is the whole lesson of reef chemistry, and everything below is detail hanging off it.

The numbers that actually matter

There are eight parameters worth tracking on a reef tank. Not thirty. Here is where each should sit, and — more usefully — what happens when it drifts.

Parameter	Target	What drift looks like
Salinity	1.025–1.026 SG	Creeps up as water evaporates. A failed top-off is the usual culprit.
Temperature	77–79°F	Swings of 2°F within a day stress fish and invite ich.
Alkalinity	8–9 dKH	Falls as corals build skeleton. Rapid swings cause tissue burn on SPS.
Calcium	420–440 ppm	Moves with alkalinity. Rarely a problem alone if alk is managed.
Magnesium	1300–1400 ppm	Low Mg makes alk and calcium impossible to hold. Check it when nothing else explains the drift.
Nitrate	2–10 ppm	Zero starves coral and triggers dinoflagellates. High feeds nuisance algae.
Phosphate	0.03–0.10 ppm	Same story. Undetectable phosphate is a problem, not an achievement.
pH	8.0–8.4	Usually a room CO2 issue. See our full pH guide.

Test on a schedule, not on a mood

The most common testing pattern in the hobby is: test obsessively for three weeks, get bored, stop, then test again in a panic when something looks wrong. By then you have a number but no trend, and a single reading tells you almost nothing. A schedule fixes this. It does not need to be demanding.

Stability beats perfection

This is the part worth tattooing somewhere. A tank held steady at 7.8 dKH will outperform a tank yo-yoing between 8 and 9.5 every week — even though the second tank spends time in the “ideal” range and the first never does. Corals are adaptable animals. They adjust their tissue chemistry and their symbionts to the conditions they are given, and once adjusted they grow happily across a surprisingly wide band. What they cannot do is re-adjust every few days.

When numbers drift, correct at the speed they drifted

Something is off. The instinct is to fix it immediately. That instinct is wrong, and acting on it has killed more corals than the original problem would have. Correct at the speed the problem developed. Alkalinity fell over two weeks? Bring it back over several days, not in one afternoon. Salinity crept up over a month of poor top-off? Walk it down across a week.

A sensible order of investigation

1. Re-test before you react. Test kits give bad readings surprisingly often — expired reagents, a rushed titration, a dirty vial. Confirm the number before you dose anything.
2. Check magnesium if alk and calcium both misbehave. Low Mg makes both impossible to hold, and people chase alk for months without checking it.
3. Check salinity next. It scales every other reading, so a salinity error looks like eight simultaneous problems.
4. Then look at consumption. A growing SPS tank consumes far more alkalinity than a softie tank. Rising demand is a sign of success, not a fault.

Get these habits right — a short list of parameters, a schedule you keep, a logbook, and gradual corrections — and reef chemistry stops being a source of anxiety. It becomes ten minutes on a Sunday. For the parameter that trips up the most reefers, read our full guide to pH in the reef tank. And if your corals look dull despite good numbers, the answer is often lighting rather than chemistry.

The Underwater Gardener grows aquacultured corals in Key West, Florida (FL Aquaculture Certificate #AQ0254072). Our corals are raised in stable, well-managed systems, so they arrive already adapted to the parameters above. Corals ship UPS Overnight every Tuesday and Wednesday.